Copolyesters of poly(hydroxyalkanoates) with aromatic dicarboxylic acids are known. For example, Gordon, et al., in WO 91/02015, disclose hydrodegradable aliphatic-aromatic copolyesters that can include hydroxy acids. Imamura, et al., in U.S. Pat. No. 5,605,981 and U.S. Pat. No. 5,616,657, disclose a process for the preparation of certain lactic acid copolyesters that comprise 98-50 weight percent lactide. Warzelhan, et al., in U.S. Pat. No. 5,817,721 and U.S. Pat. No. 5,889,135, disclose certain aliphatic-aromatic copolyesters containing hydroxy acid components. Warzelhan, et al., in U.S. Pat. No. 6,018,004 and U.S. Pat. No. 6,303,677, disclose certain aliphatic-aromatic copolyesters containing hydroxy acids. Buchanan, et al., in U.S. Pat. No. 6,342,304, disclose oriented films derived from linear aliphatic-aromatic copolyesters prepared from any polyester forming combination of hydroxy acids, dicarboxylic acids or derivatives thereof, and diols.
The introduction of a sulfonate functionality into some polyesters has been found to enhance the biodegradation rate of the polyesters and shaped articles produced therefrom.
Copolyesters of poly(hydroxyalkanoates) with aromatic dicarboxylic acids that also contain a sulfonate functionality are known. For example, Gallagher, et al., in U.S. Pat. No. 5,097,004, U.S. Pat. No. 5,171,308, and U.S. Pat. No. 5,219,646, Tietz, in U.S. Pat. No. 5,097,005, and Romesser, et al., in U.S. Pat. No. 5,295,985 disclose certain sulfonated aromatic copolyesters containing 2 to 40 weight percent of a hydroxy acid component. Warzelhan, et al., in U.S. Pat. No. 6,018,004 and U.S. Pat. No. 6,303,677, disclose sulfonated aliphatic-aromatic copolyesters containing hydroxy acids.
Known sulfonated aromatic copolyesters containing hydroxyalkanoic acid components can have unacceptable thermal properties, as evidenced by the low observed crystalline melting temperatures and glass transition temperatures. The present invention provides sulfonated aromatic copolyesters containing hydroxyalkanoic acids and having improved thermal properties, such as crystalline melting points, glass transition temperatures, and heat deflection temperatures, in comparison with known sulfonated aromatic copolyesters containing hydroxyalkanoic acids.
Various polymeric compositions have been used in an attempt to provide improved properties for uses such as sheets and wraps. In particular, poly(ethylene terephthalate) (PET) has been used to form low-cost sheets for many years. However, PET sheets can have inadequate low temperature impact strength, too low glass transition temperature (Tg) and/or too high rate of crystallization for some uses. Thus, PET sheets at low temperatures can be susceptible to breakage and diminished optical clarity at higher temperatures.
Polycarbonate sheets can be used in applications wherein low temperature impact strength is needed, or a high service temperature is required. Polycarbonate sheets have impact strengths at low temperatures and Tg suitable for use in high temperature applications. However, polycarbonate can have inadequate solvent resistance, thereby limiting its use in certain applications, and can be prone to stress induced cracking. Polycarbonate sheets also provide greater impact strength than is needed for certain applications, making them too costly and inefficient for some uses.
A need remains for polymeric materials that can be used in making articles such as packages, and that have thermal properties that enable them to be used in higher temperature uses without the loss of physical and structural integrity.